The present invention relates to diene rubber compositions usable for the manufacture of tires or semi-finished products for tires, in particular treads for tires, and to reinforcing fillers capable of reinforcing such rubber compositions.
In order to reduce fuel consumption and pollution emitted by motor vehicles, major attempts have been made by tire designers to obtain tires having all of the following characteristics: very low rolling resistance, improved grip, both on dry ground and on wet or snow-covered ground, and very good wear resistance.
Numerous solutions have been proposed to lower the rolling resistance and to improve the grip of tires, but these generally result in a very great decline in wear resistance. It is known that the incorporation of conventional white fillers such as conventional silicas or aluminas, chalk, talc, titanium oxides, clays such as bentonite or kaolin in rubber compositions used in the manufacture of tires, in particular treads, results in a reduction in the rolling resistance and an improvement in grip to wet, snow-covered or icy ground. However, such compositions also result in an unacceptable decline in wear resistance, because such conventional white fillers do not have a sufficient reinforcement ability in such rubber compositions. Thus, these white fillers are generally referred to as non-reinforcing fillers or, alternatively, inert fillers.
One solution to this problem was described in Patent Application EP-A-0 501 227, which discloses a diene rubber composition reinforced by a special precipitated silica (SiO2) which allows manufacture of a tire having substantially improved rolling resistance, without adversely affecting the other properties, particularly grip, endurance and wear resistance. European Patent Application EP-A-0 810 258 discloses a diene rubber composition reinforced by another special white filler, a specific alumina (Al2O3) of high dispersibility, which allows production of tires or treads having an excellent compromise of contradictory properties.
Because of these new white fillers, referred to as reinforcing fillers, it has also been possible to begin commercializing colored tires, in particular, those with colored treads. For aesthetic reasons, the ability to produce such tires meets a real expectation of users, in particular for passenger vehicles while, at the same time providing a substantial saving in terms of fuel.
Applications WO99/02590 and WO99/06480 disclose rubber compositions, based on reinforcing silicas or aluminas, used for manufacturing tires having treads or sidewalls of different colors. One or more inert fillers (i.e., non-reinforcing ones), such as kaolin, talc or titanium oxide, that function as a pigment or pastelizing agent are used as complementary white fillers in these compositions.
Among inert fillers, titanium oxides, in particular, are well-known as white pigments in different matrices such as paints, inks, cosmetics, plastics materials and polymers, such as rubber compositions intended to be incorporated in tire sidewalls (cf. for example CA-A-2054059, CA-A-2058901, CA-A-2228692, GB-A-836716, EP-A-697432, and applications JP1991/006247, JP1995/149950 and JP1996/059894).
In addition to their pigmentation or pastelizing properties, titanium oxides also are effective anti-UV agents, that can provide beneficial anti-aging protection of colored rubber compositions. Such compositions are normally devoid of carbon black (which is an excellent UV absorber) and thus are sensitive to the degrading action of sunlight (see applications WO99/02590 and WO99/06480 referred to above). Moreover, they cannot be protected by antioxidants (paraphenylenediamine type) conventionally used in black tires, because of the staining effect of these antioxidants.
During the course of research, the inventors have found that there are special titanium oxides which may be used, not only as anti-aging or pigmentation agents, but as true reinforcing fillers capable of replacing conventional carbon blacks in rubber compositions for tires. These titanium oxides unexpectedly represent an advantageous alternative to the use of reinforcing silicas or aluminas, in rubber compositions, particularly colored rubber compositions. In other words, compositions can comprise a single filler, a titanium oxide, which has several functions (reinforcement, pigmentation, anti-aging), instead of several fillers where each has a single function.
The invention relates to a rubber composition usable for the manufacture of tires, comprising, as base constituents, (i) one or more diene elastomer(s), (ii) a white filler as reinforcing filler and (iii) a coupling agent (white filler/elastomer) linking the reinforcing filler and the elastomer, wherein the white filler comprises a titanium oxide having the following characteristics:
(a) it comprises more than 0.5% by mass of a metallic element other than titanium, selected from the group consisting of Al, Fe, Si, Zr and mixtures thereof;
(b) its specific BET surface area is between 20 and 200 m2/g;
(c) its average particle size (by mass), dw, is between 20 and 400 nm; and
(d) its disagglomeration rate, a, measured by the ultrasound disagglomeration test, at 100% power of a 600-watt ultrasonic probe, is greater than 2xc3x9710xe2x88x922 xcexcmxe2x88x921/s.
The prior art neither describes nor suggests the titanium oxide as described above in a rubber composition as reinforcing filler. The above titanium oxide is referred to as a xe2x80x9creinforcing titanium oxidexe2x80x9d, which, without anything other than an intermediate coupling agent (white filler/elastomer) can reinforce rubber compositions usable for the manufacture of tires, in particular treads of tires, having a high wear resistance. In particular, the prior art, e.g., CA-A-2054059, CA-A-2058901 or CA-A-2228692 referred to above, teaches the non-reinforcing nature of titanium oxides and the necessity of adding a reinforcing filler, such as carbon black or silica, to give a minimum level of reinforcement to the rubber compositions described.
Another aspect of the invention is the use of a rubber composition according to the invention for the manufacture of rubber articles, in particular tires or semi-finished rubber products intended for such tires, these semi-finished articles selected from the group consisting of treads, underlayers intended to be positioned beneath the treads, crown plies, sidewalls, carcass plies, beads, protectors, inner tubes and airtight internal rubbers for tubeless tires. The composition of the invention is particularly suitable for the manufacture of sidewalls or tire treads for tires intended to be fitted on passenger vehicles, vans, 4xc3x974 vehicles, two-wheelers and heavy vehicles, aircraft, construction machinery, agricultural machinery or handling machinery. The treads may be used in the manufacture of new tires or for recapping worn tires.
Another subject of the invention are the tires and semi-finished rubber products themselves comprising a rubber composition according to the invention.
A further subject of the invention is the use of a reinforcing titanium oxide for reinforcing a diene rubber composition usable for manufacturing tires.
A still further aspect of the invention is a process for reinforcing a diene rubber composition usable for the manufacture of tires, comprising incorporating a reinforcing titanium oxide into the diene rubber composition in the uncured state by thermo-mechanical kneading.
The rubber compositions of the invention are particularly suited for the manufacture of colored tires or colored semi-finished articles, such as treads or sidewalls. xe2x80x9cColoredxe2x80x9d rubber compositions, tires or rubber articles in the present description are understood to be rubber compositions, tires or articles, at least part of which is of a color, including white, other than the conventional black.